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Roger Shepard
| | Posted on Thursday, November 08, 2001 - 1:20 am: | 
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I'm getting ready to put a 3 element vertical beam and I would like to fine tune the swr. How is the best way? I have them on saw horses in the garage, of course laying down makes them horizontal but I dont see the difference. Is this as good as standing them up? If I stand them up, I will no longer be able to reach the adjustments. I have even connected the first section of tower to the mast pole to simulate being installed. I hooked them up and did not get a high swr, but because my coax is on the concret floor Im having conflicting readings when I move the coax. Im using mini 8, with a 97% sheild. Should I use super 8 for this purpose, I have a small amount. Any help would be useful. I am using a power scanner pdc700 for a swr meter. |
Marconi
| | Posted on Thursday, November 08, 2001 - 12:41 pm: |
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Roger, I am not getting a good picture of what you are doing in the garage. You say you have installed the tower section and mast pole with the antenna to reflect the presence of the mounting material. This is a good idea, but how are you able to do this in the garage? You also express some concerns about not being able to reach the tuning area of the antenna. You are just going to have to figure out a way to do this while being careful in doing so.
Tuning in the garage is a bad idea from the get-go. Tuning near the area you will install the antenna would be better. Tuning at installed height would be the best. That is what I do. Tuning on sawhorses, close to the earth, will not give you reliable readings for end results. Some will say tune the beam, base down, and point it towards the sky, that will give you good results. It may work. I have never tried it. I have watched others do it and in the end, further tuning at installed height seemed necessary in the end. Most will say tune it at least 9’ feet above the earth and tune the vertical in the horizontal plane at least 9’-10’ feet above the earth. Still others will say tune to channel one and when you go up with the antenna your tune will fall in somewhere around center of CB band. You can tune at a minimum height around 9’ feet or more and the antenna should work just fine right there. However, when you raise it up you will likely see the need to retune as your SWR may change drastically. This will likely be due to a matching change that happens as you go higher.
If you go vertical, I would suggest you make the mast long enough to get the antenna above the top of the tower. This is why I go horizontal only. I do not have to deal with these problems and the lack of headroom.
I am curious, assuming you have a single polarity beam, why are you electing to go vertical with your beam? If it is a local contact thing then it is understandable.
BTW, what brand of beam do you have? If it is a Maco M-103, I think following their docs closely will probably put you real close. They give you good optimized measurements relative to a specific frequency range and that is nice to have. It has been my experience that you can go by their measurements and just about be on mark every time.
When you refer to the coax, I assume you are talking about the line you intend to install when erected. What are you doing with all the extra coax while testing? When this close to the earth, ground losses will dramatically show their effects and much of your feed line is bound to be in the near field of your antenna. This may be why you are seeing variations in your previous readings.
Try tuning using a very short piece of coax or use a tuned and resonant electrical ½ wave line in this case. If you know how to plot a “frequency/SWR” curves, then I suggest you do that while keeping good records as you go. This will give you a better handle on what the SWR readings mean to your tuning efforts. Lower SWR is not always better.
Personally, I tune for effect using a responsive field strength meter. This is tedious work but it is not as suspect as simply getting a low SWR. Of course I read SWR as I go along, I make bandwidth curves to see where I am in the process. I don’t get in a hurry and often I will make actual response checks on air. I make both TX/RX but I depend mostly on RX signals. This provides me a feel for how the antenna is doing in the real world. If I don’t seem to get good rejection off the back and sides and a good directional signal improvement from known local stations, then I continue to tune. I do this type of checking during quite times only. You may ask how do you do this! Simple, you must keep good records of your measurements and results with all of your tuning efforts. Do a little homework with your omni antenna beforehand, again keeping good records. The more you fiddle with these antennas and their responses the more you will learn and understand about their characteristics.
The three element yagi antenna is a very effective antenna when properly tuned. It is fairly simple to tune and is a perfect experience builder. Keep up the good work and good luck.
Marconi |
Roger Shepard
| | Posted on Saturday, November 10, 2001 - 12:31 am: |
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Thanks for the info. Marconi. But your tech. level is way above mine. I sure would like to have those measurments you were talking about. I exspanded the two outside elements out about one inch further apart, by the way, my ant. prongs go up and down. I never was really good with Horizontal and Vertical. As far as the garage thing goes I have a really large garage attached to my home and I assembled the entire ant. together in my garage including mast poll and one section of tower and still had plenty of room to work and park a van. The prongs on the ant. are 19'6" long and the boom is 12', and they are astro beams from the 70's which I bought new back them. I came to the idea that the ground was effecting my coax. As far as why I had so much coax, I figured in order to get the best tune you would need to use the equipment that was going to be used once they were installed as to be sure everthing was in working order. I have since removed the ant. from the garage and stood them up on my deck which is 3 feet off the ground to give more distance from earth, but I still cant reach the gamma match. I use a long mast poll so that the ant. will not come down along side of the tower except about 2 feet and I have my rotor mounted way down in the tower for better support and thats why I have such a long mast pole. You could not even adjust them once they were up because of the long mast. The live tuning you were talking about seems nearly impossible unless you have a good friend who is willing to work with you so that you would have a constant signal. I really dont know much about tuning an ant. except for swr. tuning for freq. is out of my leage. I would like to hear more form you and others on this subject, because all help is needed. I hoped the techs. in the forum would also offer advice as well. thanks a lot. Roger |
marconi390@pdq.net
| | Posted on Saturday, November 10, 2001 - 12:03 pm: |
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Roger, you have a big garage! The AstroPlane antenna does not have a gamma match device. It is referred to as a 5/8's wave ground plan, but the best I can see is, it is a modified center fed 1/2 wave dipole that uses a capitacance hat to add length to the stinger. I think the only tuning for this antenna has to do with tuning for resonance, by adjusting the element lengths and maybe a little adjustment in the cap hat.
Another tip to try would be to run the coax down through the mast pipe just in case you are getting RF on the outside of the feed line. A simple field strength meter would help you determine if this is happening.
From what you describe, the top of the tower must go up inside the circular ground hoop at the bottom of the antenna. I have no idea what affect this would have but suspect it is probably not good. If you have a pushup pole that you are referring to as a tower then that is another thing all together. That should not be a problem as the antenna was designed with the mast pipe in place. But a tower is generally much larger and can present a different effect.
In my minds eye I see no way to mount this antenna on the horizontal without modifying the mounting bracket. If you did, I think the weight of the ground hoop would cause it to sway execssively towards the ground, as it is not supported in that manner.
Do you have the documentation that came with the Astro Beam antenna?
Adding an inch to the spacing of the two outer elements may not do any harm, but why did you consider extending the spacing?
I am not sure about which measurements you are requesting, but if you are referring to my M-103, the measurements will do you no good. You need to go by the measurements noted by Avanti who designed the AstroPlan.
Marconi |
Marconi
| | Posted on Sunday, November 11, 2001 - 12:05 am: |
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Roger, drop me an email and I will send you some information about tuning using an SWR meter.
Just click on my handle to the left.
Marconi |
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